The present invention relates to a compound roll composed of a shell and a core, and more particularly to a compound roll composed of a high-chromium cast iron shell and a cast or forged steel core.
Rolls for hot and cold rolling are conventionally formed from alloy cast iron or hardened forged steel, but they suffer from various problems such as low resistance to wear and failure. For the purpose of improving the hardness of roll shells, high-chromium cast iron compound rolls were developed. A typical high-chromium cast iron compound roll is composed of a high-chromium cast iron shell and a cast iron or spheroidal graphite cast iron core. It was manufactured by a centrifugal casting method. See J. Honda et al.,"Compound Cast Rolls for Steel Rolling Mills," IMONO (Casting), Vol. 54, pp. 44- 50, 1982; H. Muller et al., "High-Chrome Work rolls in a Modern Hot Strip Mill," Iron and Steel Engineer, pp. 63-70, Oct. 1975; and M. Grounes, "New Roll Types with Superior Performance," Iron and Stee Engineer, pp. 42-49, April 1979.
Particularly for hot rolling, work rolls have been increasingly required to have high resistance to wear, failure, adhesion of rolled materials and surface roughening from the viewpoint of improving rolling operation and efficiency. For these purposes, high-chromium cast iron roll appears to be promising.
And for cold rolling, much higher stress is applied to the compound rolls in operation, so that the cores of the compound rolls should have extremely good mechanical properties.
In addition, conventional four high mills comprising a pair of work rolls and a pair of back-up rolls, either for hot rolling or for cold rolling, have been increasingly replaced by six high mills having intermediate rolls between work rolls and back-up rolls, or mills having work rolls which can be shifted, in order to apply higher pressure to metal sheets to be rolled. Because an extremely high load is applied to the work rolls, the maximum contact pressure of the work rolls can reach, for instance, up to 240 kg/mm.sup.2 as compared with 160 kg/mm.sup.2 for the four high mills. As a result, spalling has become a serious problem.
At the same time, in such high-pressure mills, a larger bending force is applied to the shafts of the work rolls, so that the roll shafts have been required to have higher mechanical strength.
For such purposes, a conventional centrifugal casting method has turned out to be unsatisfactory, because it failed to provide compound rolls having sufficiently hard shells and sufficiently tough cores. Specifically, high-chromium compound rolls manufactured by the centrifugal casting method had inevitably cast cores, and the cores' mechanical properties were lower than expected. For instance, even with spheroidal graphite cast iron, the cores had tensile strength of 35-55 kg/mm.sup.2 and elongation of 0.2-0.5%. It has turned out that the deterioration of mechanical properties of the cores is caused by diffusion of chromium contained in the shells into the cores during the casting of molten core materials, and that this phenomenon is unavoidable in the centrifugal casting method. Accordingly, extremely high rolling pressure could not be achieved with high-chromium compound rolls manufactured by the centrifugal casting method.
As for the shells, although a proper heat treatment can increase Shore hardness of high-chromium cast iron, the shells of the high-chromium compound rolls manufactured by the centrifugal casting method can reach Shore hardness of only 80-90 at highest. The reason therefor is that since a hardening treatment accompanied by rapid cooling for martensitic transformation may lead to the breakage of the rolls, tempering has to be carried out several times to decrease a large amount of residual austenite, inevitably causing the softening of the shells by tempering to a large extent. Thus, the high-chromium cast iron compound rolls manufactured by the centrifugal casting method could not attain sufficient wear resistance.
In view of the above problems with the centrifugal casting method, a method of forming a shell around a core by casting a shell material around the core was recently developed.
U.S. Pat. No. 3,455,372 issued to Yamamoto on July 15, 1969 discloses a continuous padding method using high frequency current. This method comprises preheating the surface of a core material by moving the core material up and down through a mold assembly composed of a heating mold, a buffer mold and a cooling mold, and after returning the core material to a predetermined position, moving it downwardly and slowly through the mold assembly while pouring a melt of padding material into the gap between the core material and the mold assembly, whereby the melt is bonded to the surface of the core material, cooled to some extent within the buffer mold, and further cooled and solidified rapidly within the cooling mold to form a layer of pad on the surface of the core material.
This method, which may be called simply"shell casting method," can provide a compound roll composed of a hard shell and a tough core. High-chromium cast iron compound rolls manufactured by the shell casting method are subject to heat treatment. However, a usual heat treatment comprising hardening and tempering fails to achieve the maximum properties which these compound rolls potentially have. Particularly, spalling remains to be a serious problem for the high-chromium cast iron compound rolls thus manufactured.
Spalling is a fatigue type of failure which can be caused by severe mechanical stress. Most defects of this type are greater at depth than at the surface. In most of these cases, the failure is fatal, with a crack extending to the shell/core interface.